A device for ultrasonic sealing

Abstract: The invention relates to a device for ultrasonic sealing, of compact constructional dimensions, consisting of a drive unit (1), a horn (7) with a long and narrow sealing surface (8), and one or more reaction bodies (9). The length of the device consists of a half wave length.

Description: TECHNICAL FIELD The present invention relates to a device for ultrasonic sealing, comprising a drive unit connected to an A.C. current source, and a horn. BACKGROUND ART Conventional devices for ultrasonic sealing normally consists of a converter a booster and a horn. In the converter, an electric oscillation is converted into a mechanical oscillation, the booster is employed to fix the system mechanically and boost the amplitude. In the horn, further amplitude boosting takes place, at the same time as the horn provides the desired sealing surface. These conventional, thoroughly well-tested ultrasonic sealing devices suffer, however, from the disadvantage of being relatively bulky. For certain practical applications, it is desirable to realize an ultrasonic sealing device which requires considerably less space. Within the packaging industry which deals with packages of the single-use disposable type, it is common to seal laminates with thermoplastic or pure thermoplastic material. These seals are normally carried out using heat or, when the laminate includes an aluminium foil, using induction heat. However it is desirable to be able to employ ultrasonic sealing, since this method does not require a laminate which includes a metal foil. The conventional ultrasonic sealing devices have however, proved to be far too bulky. A device for ultrasonic sealing, is known from US-A-4 995 938, disclosing an assembly consisting of an ultrasonic vibrator generating ultrasonic vibration and is shaped as a column. An ultrasonic vibration transmitter is coupled to the bottom of the ultrasonic vibrator coaxially thereto. A tool 3 is coupled to the bottom of the transmitter so as to perform machining by using ultrasonic vibration. However, this assembly constitutes one wave length or longer. Small-scale ultrasonic device sealing equipment has already been produced. However, these are rotation-symmetric and can only carry out spot-sealing. In the above-outlined application within the packaging industry, it is desirable to carry out transverse sealings of a certain length, i.e. a long and narrow seal. OBJECT OF THE INVENTION One object of the present invention is to realize an ultrasonic sealing device which is of considerably smaller dimensions than conventional equipment and which is capable of executing a long and narrow sealing joint. SOLUTION This and other objects have been attained according to the present invention in that the device of the type described by way of introduction has been given the characterizing features of claim 1. Preferred embodiments of the present invention have further been given the characterizing features as set forth in the appended subclaims. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS One preferred embodiment of the device according to the present invention will now be described in greater detail hereinbelow, with particular reference to the accompanying Drawings. In the accompanying Drawings: Fig. 1 is an outline diagram showing a device for ultrasonic sealing; Fig. 2 shows one embodiment of the device according to the invention; Fig. 3 shows yet a further embodiment of the device according to the present invention; Fig. 4 shows still a further embodiment of the device according to the present invention; and Fig. 5 shows the principle for the amplitude of the device. DESCRIPTION OF PREFERRED EMBODIMENT To be able to employ ultrasonic sealing in filling machines, a compact sealing device is necessary in which a uniform oscillation amplitude is maintained at the entire sealing surface so that an even and dependable seal along the entire transverse seam or joint will be obtained. Fig. 1 shows the principle of an ultrasonic sealing device according to the invention, which has compact constructional dimensions and is of a design which makes possible a uniform and efficient sealing along the entire sealing surface. The device consists of a drive unit 1 of conventional type which generates the oscillation. In the preferred embodiment, the drive unit consists of piezoelectric ceramic plates 2 which are disposed with interjacent conductive metal sheets 3, preferably manufactured from nickel or beryllium copper. The drive unit 1 moreover consists of a counter weight 4 and the entire arrangement is mounted on the device by means of a clamping bolt 10. Alternatively, a magnetostrictive drive unit 1 may be employed. The drive unit 1 is coupled to an A.C. current source (not shown). The plane beneath the drive unit 1 constitutes the nodal plane 5 of the device. The mechanical fixation of the system takes effect herein and the slightly bevelled edge 6 simultaneously serves as fixation edge and makes possible securement of the device. The securement of the device should be resiliently yieldable so as to compensate for certain residual oscillations in the nodal plane. Beneath the nodal plane 5, a horn 7 is located in which the amplitude boosting takes place. The horn 7 provides the device with its desired sealing surface 8. The shape of the horn 7 should be tapering towards the sealing surface 8. This shape may, for example, be obtained in that the horn 7 is reduced, from its largest surface of extent at the nodal plane 5, with a curved side surface 11 to the sealing surface 8 as shown in Fig. 4. Alternatively, this configuration may be made to be exponential-like, i.e. be exponential or catenoid. The sealing surface 8 is preferably long and narrow in order to be able to achieve the desired transverse seal on a package blank. The sealing surface 8 is suitably provided with one or more grooves where the actual heating against the material takes place. Alternatively, the back-up surface (not shown) may be provided with grooves, while the sealing surface 8 is smooth. Yet a further alternative is to divide up the sealing surface 8 into two narrower mutually discrete but parallel sealing surfaces 8. Like the fixation edge 6, the horn 7 is manufactured from a light metal or light metal alloy, preferably aluminum, but other metals such as titanium may also be employed. The device further includes reaction masses, so-called reaction bodies 9 which surround the drive unit. These reaction bodies 9 are similarly manufactured from a light metal, preferably aluminium, so that the horn 7, the fixation edge 6 and the reaction bodies 9 constitute a homogeneous block. Alternatively, these parts may be manufactured separately and subsequently assembled. The reaction bodies 9 may be of different configurations, but should be as large as possible and, in the preferred embodiment according to Fig. 2, they are therefore provided with a gently rounded surface facing the drive unit 1. The reaction bodies 9 may be of exponential configuration which is similarly rounded facing the drive unit 1, as shown in Fig. 3, or a curved side surface 12 as shown in Fig. 4. The reaction bodies 9 may also be designed as a completely continuous mass which wholly or partly surrounds the drive unit 1. By combining, in an appropriate manner, the configuration of the horn 7 and the design of the reaction bodies 9, the intention is to obtain a resonance frequency which is as pure as possible, without disrupting elements from other resonance frequencies in the proximity of the frequency chamber. The above-disclosed ultrasonic sealing device according to Figs. 1 - 3 realizes a long and narrow sealing which corresponds to the length of the sealing surface 8. When a longer sealing surface 8 is desired, a plurality of ultrasonic sealing devices according to the invention may be interconnected for additional length. Alternatively, longer devices may be manufactured, which then require slits in the horn 7 so as to compensate for the lateral oscillations which occur. However, a longer horn 7 gives a more uneven amplitude at the sealing surface 8, which may result in an uneven seal. When the device according to the invention is connected to the A.C. current source (not shown) via the drive unit 1, the piezoelectric ceramic pack 2 converts the electrically supplied voltage into a mechanical strain which supplies energy to the horn 7 which, in such instance, begins to oscillate. When the horn 7 begins to oscillate, counter forces (counter oscillations) occur which are assimilated by the reaction bodies 9. In this instance, a nodal plane 5 occurs between these oscillations which, in principle, consists of the fixation edge 6 of the device. Fig. 4 shows the amplitude of the device. The device constitutes a half-wave resonator with a centrally located nodal plane 5 where the amplitude is equal to 0. The entire length of the device thus consists of half a wave length. The device is dimensioned so that a common nodal plane 5 occurs for each point in the device. The reaction bodies 9 must also be designed so that no undesired oscillations occur. As will have been apparent from the foregoing description, the present invention realizes a device for ultrasonic sealing which constitutes a compact unit which can execute a long, narrow and uniform seal. The present invention should not be considered as restricted to that described above and shown on the Drawings, many modifications being conceivable without departing from the scope of the appended Claims.

Claims: A device for ultrasonic sealing, comprising a drive unit (1) connected to a A.C. current source, and a horn (7), characterized in that the device further includes one or more reaction bodies (9), attached to the horn (7) at a nodal plane (5) and placed so as to surround the drive unit (1) and that the reaction bodies (9) together with the horn (7), constitute a half wave length. The device as claimed in Claim 1, charaterized in that the drive unit (1) consists of piezoelectric ceramic plates (2). The device as claimed in Claim 1, characterized in that the drive unit (1) consists of a magnetostrictive unit. The device as claimed in one of the preceding claims, characterized in that the horn (7) has a long and narrow sealing surface (8). The device as claimed in Claim 4, characterized in that the horn (7) has, from its largest extent, at the nodal plane (5) to the sealing surface (8), a curved side surface (11). The device as claimed in Claim 4, charcterized in that the horn (7) has exponential or exponential-like configuration. The device as claimed in Claim 4, characterized in that the horn (7) has catenoid configuration. The device as claimed in one of the preceding claims, characterized in that the reaction bodies (9) have a curved surface (12). The device as claimed in one of the preceding claims, characterized in that the reaction bodies (9) are of substantially parallelepipedic configuration which is somewhat concave towards the drive unit (1). The device as claimed in one of the preceding claims, characterized in that the reaction bodies (9) are of exponential configuration which is somewhat concave towards the drive unit (1). The device as claimed in one of the preceding claims, characterized in that the nodal plane (5) constitutes a mechanical fixation point. The device as claimed in one of the preceding claims, charcterized in that the horn (7) and the reaction bodies (9) are of light metal. The device as claimed in Claim 12, characterized in that the light metal consists of aluminium.

Inventors: OLSSON HAKAN, RABE MAGNUS, OLSSON, HAKAN, RABE, MAGNUS
Applicants: TETRA LAVAL HOLDINGS & FINANCE, TETRA LAVAL HOLDINGS & FINANCE SA
Application year: 1994
Main classification: B65B51
IPC codes: B29C65, B23K1, B06B3, B65B51